This disclosure generally relates to systems and methods for balancing rotating machinery to reduce or minimize vibrations. In particular, the disclosed embodiments relate to systems and methods for balancing gas turbine engines.
It is either impossible or nearly impossible, as a practical matter, to build a rotating structure that is perfectly balanced upon manufacture. Any such structure will produce a certain amount of undesired vibration to a greater or lesser extent. Such vibration is usually passed through bearings that support the rotating part of the structure, and can therefore manifest itself as unwanted noise or vibration in adjacent structures. As is known to those skilled in the art, synchronous vibration may be characterized by an amplitude (i.e., magnitude) and a phase angle (i.e., direction). Thus, the vibration of a part may be represented as a vector or phasor.
One type of rotating machinery susceptible to undesired vibration is the high-bypass turbofan engine used in commercial aviation. Such engines have a large number of rotating elements. These rotating elements can be grouped according to the relative speed of rotation. Some of the rotating elements form a low-speed rotating system and other rotating elements form one or more high-speed rotating systems. More specifically, each rotating system of a gas turbine engine comprises an upstream rotating multi-stage compressor connected to a downstream multi-stage turbine by means of a shaft. The low-pressure turbine and low-pressure compressor are connected by a low-pressure shaft; the high-pressure turbine and high-pressure compressor are connected by a high-pressure shaft which surrounds a portion of the low-pressure shaft, with the high-pressure compressor and turbine being disposed between the low-pressure compressor and turbine. The fan of the turbofan engine is the first stage of the low-pressure compressor. Vibration caused by unbalances in the various stages of a turbofan engine contributes to wear and fatigue in engine components and surrounding structures, and unwanted noise in the passenger cabin of the airplane.
One way of reducing structurally transmitted vibrations is to balance the rotating systems of aircraft engines on an individual basis. Engine balancing is well known in the aircraft art. The manufacturers of turbofan engines have developed techniques for controlling the magnitude of unwanted vibration by affixing balancing mass to the engine. Typically, the fan and the last stage of the low-pressure turbine of a turbofan engine are the only accessible locations for applying balancing mass after the engine is manufactured or assembled. Internal stages are inaccessible as a practical matter.
A known method for applying balancing mass involves the selection of a combination of balancing screws from a set of screws of different standard mass, with screws being threadably inserted into respective threaded holes located around an outer periphery of an internal turbofan engine component (such as a fan spinner). For example, to achieve a balance, one or more screws of the same mass or different masses can be screwed into respective threaded holes, thereby producing a center of gravity which is closer to the axis of rotation than was the case without balancing. The total effect of multiple attached balancing masses can be determined by treating each mass and its respective location as a vector, originating at the axis of rotation, and performing a vector sum.
The specification of the location and amount of mass to be applied to a rotating system in order to balance it is referred to herein as the balance solution for the rotating system. In order to determine balance solutions for rotating systems of turbofan engines, vibration data is obtained. Vibration data is a measure of the amount of vibration that an engine is producing at various locations as the engine is operated at various speeds and through ranges of other parameters. Vibration data can be gathered at an engine balancing facility located on the ground or during flight. If accelerometers are used to capture rotating system vibration response, synchronous vibration data may be derived using a keyphasor index on the rotating system. While multiple methods known to the art can be used to capture and derive vibration data, that data should contain a displacement as well as a phase corresponding to synchronous vibration. After vibration data is obtained, the vibration data, measured at the accelerometers, and sensitivity of the accelerometers to unit weights applied at the balance locations are used to derive a balance solution that attempts to minimize the vibration of the engine producing the data.
Jet engines generate vibratory loads due to inherent imbalance on each stage of the rotating shafts which transfer through bearings to propulsion stationary structures and are transmitted through wing structures to emerge as cabin noise and vibration. Airframe manufacturers seek to keep cabin noise and vibration at low levels to ensure passenger and crew ride comfort. Typically, airframe and engine manufacturers agree on the level of engine vibration limits measured during engine runs on a test cell at two sensors before shipment to the airframe manufacturer for installation on an airplane. Normally, engine vibrations during airplane ground runs are close to the levels measured on a test cell. However, engine vibrations during flight can be much higher than ground-run levels, causing excessive cabin noise and vibration. Higher engine vibrations during flight may be caused by extra fan imbalance due to fan blade movement during flight, especially for a new generation of engines with wide-chord fan blades. As a result, subsequent flight tests may be needed and engine balancing may be performed after each one until acceptable cabin noise and vibration levels are obtained. Each flight test costs significant time and money to conduct.
It would be beneficial to provide a balancing approach which reduces the number of flight tests and balancing operations caused by the foregoing issues.